Various embodiments of lenses of variable focal length are described in commonly owned European Patent No. 1,166,157. FIG. 1 of the current application is similar to FIG. 12 of that patent. A cell is defined by two transparent insulating plates 1 and 2 and side walls (not depicted). The lower plate 2, which is non-planar, comprises a conical or cylindrical depression or recess 3, of axis Δ, which contains a drop of an insulating liquid 4. The remainder of the cell is filled with an electrically conductive liquid 5, non-miscible with the insulating liquid, having a different refractive index and essentially the same density. An annular electrode 7, which is open facing the recess 3, is positioned on the rear face of the lower plate 2. Another electrode 8 is in contact with the conductive liquid 5. Through electrowetting phenomena it is possible to modify the curvature of the interface between the two liquids, according to the voltage V applied between the electrodes 7 and 8. For example, the curvature changes from the concave initial shape denoted by the reference A to the convex shape depicted in dotted line and denoted by the reference B. Thus, a beam of light passing through the cell normal to the plates 1 and 2 in the region of the drop 4 will be focused to a greater or lesser extent according to the voltage applied. The conductive liquid is typically an aqueous liquid, and the insulating liquid is typically an oily liquid.
The mount for the lens formed by the transparent plates 1, 2 and the side walls connecting the transparent plates generally constitutes a rigid structure. The pressure of the liquids in the lens mount may increase substantially, for example, during operations of assembling the parts that make up the mount or, once the mount has been assembled, when the liquids of the lens, which have coefficients of expansion higher than the coefficients of expansion of the materials of which the mount is made, are subject to temperature increases.
An excess of pressure of the liquids contained in the lens carries the risk of causing deformation of the upper and lower transparent plates 1, 2, resulting in undesirable optical distortion. At worst, if the liquid pressure increase is too great, it may cause the transparent plates 1, 2 to rupture. It is therefore necessary to take precautions when assembling the lens and/or to limit the permissible temperature range for storage and use of such a lens.